Download The difference between voltage and potential difference

INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Slavko Vujević1, Tonći Modrić1 and Dino Lovrić1
1University
of Split,
Faculty of electrical engineering, mechanical engineering and
naval architecture
Split, Croatia
The difference between voltage and
potential difference
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Introduction (1)
 different definitions of potential difference, voltage and electromotive force
→ confusion with some basic notions
 there is a difference between voltage and potential difference,
depending on what is our observation point
 static electric fields → conservative fields → the electromotive force for
any closed curve is zero
 time-varying electric field is not a conservative field → the electromotive
force induced in the closed curve can be expressed in terms of partial time
derivative of the magnetic flux and it is different from zero
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Introduction (2)
 what voltmeter measures, whether the position of observed points or
position of the voltmeter leads affects the voltmeter readings?
 transmission line model → the voltage depends on the path of integration
 transversal voltage is a special case of voltage equal to the potential difference
 electrical circuit analysis → branch voltages are unique and equal to
difference of nodal voltages (nodal potentials)
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Static fields (1)
 static fields do not change with time → the simplest kind of fields
 electrostatic fields → produced by static electric charges
 stationary currents → associated with free charges moving along closed
conductor circuits
 magnetostatic fields → due to motion of electric charges with uniform
velocity (direct current) or static magnetic charges (magnetic poles)
 the electric field generated by a set of fixed charges can be written as
the gradient of a scalar field → electric scalar potential φ

E  electric field intensity
  electric scalar potential

E  
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Static fields (2)
 unique voltage uAB can be defined for any pair of points A and B
independent of the path of integration between them
B
 
u AB   E  d    A   B ; Ci
A
 Stokes theorem → Maxwell equation for static electric fields:


 
 
 E  d      E  dS  0
Ci

 E  0
Si
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Static fields (3)
 the work done on the particle when it is taken around a closed curve is zero,
so the voltage around any contour Ci can be written as:
u AA
 
 E  d    A   A  0 ; Ci

Ci
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Time-varying fields (1)
 can be generated by accelerated charges or time-varying current



 
dB
B
 E  

  v  B
dt
t


→ Maxwell equation for time-varying fields

v  relative velocity beetween magnetic field and medium

B  magnetic flux density


B  A


A  
E    
vB
t
 

E  E stat  Eind

E stat   




A  
Eind  Etr  Em    v  B
t

A - magnetic vector potential
→ the electric field intensity for time-varying fields
→ total electric field intensity
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
 Closed curves:
Time-varying fields (2)
 




u   E  d    Estat  d    Eind  d 
Ci
Ci

0
Ci


eetr em
e  induced electromot ive force
 for any contour Ci, voltage u is equal to induced electromotive force e:
u CAAi

e CAAi
 


 E  d   Eind  d 

Ci

Ci
voltage and induced electromotive force depend on the integration path
 transformer electromotive force, etr, can be expressed as negative of partial time
derivative of the magnetic flux Φ through the contour Ci over the surface Si:

etr  
t

Ci
 

A  d  
t

Si
 

B  dS  
t
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Time-varying fields (3)
 Open curves: voltage between any pair of points A and B can be defined as:
B
u AB
  B

  E  d    Estat  d  
A
A 


 A  B
B


 Eind  d 
A 


e AB etrAB emAB
u AB   A   B  e AB
 difference between time-varying voltage and potential difference is evident
and these two concepts are not equivalent
 potential difference between any two points is independent of the integration path
 voltage and induced electromotive force between any two points are not equal
and depend on the integration path
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
AC voltmeter reading (1)
 conventional circuit analysis without time-varying fields → Ohm law and
Kirchhoff voltage law
 time-harmonic electromagnetic field → Ohm law and Kirchhoff voltage law
extend with Faraday law
 the voltmeter readings are path dependent
 the measured voltage depends on the rate of change of magnetic flux through
the surface defined by the voltmeter leads and the electrical network
 time-harmonic electrical network currents and current through the voltmeter,
connected between points A and B, will induce a transformer electromotive force:
   j   
  phasor of the induced electromotive force
  phasor of the magnetic flux through the contour
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
AC voltmeter reading (2)
 Thevenin equivalent consists of Thevenin electromotive force and Thevenin
impedance and represents the electrical network between points A and B
→
 Thevenin electromotive force ET, induced electromotive force ε, magnetic flux Φ
and current through the voltmeter are phasors with magnitudes equal to effective
values
 voltmeter reading is equal to effective value of voltage on voltmeter impedance
UV  UV  I  ZV
ET  

 ZV
ZT  ZV  Z L
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Transmission line model (1)
 two-conductor transmission line model → voltage u and current i along the line:

u
i
 Ri  L
x
t

i
u
Gu  C 
x
t
 in time-varying electromagnetic field, voltage between two points depends on
integrating path
4
u14
 
 E  d   1   4  u

1
3
 
u
u 23  E  d    2  3  u 
 dx
x

2
 transversal voltage is a special case of voltage equal to the potential difference
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Transmission line model (2)
 single-conductor representation of the two-conductor transmission line of length ℓ,
with uniformly distributed per-unit-length parameters R, L, C and G:
 transversal voltages u1 and u2 are equal to the potentials φ1 and φ2
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Electrical circuit theory (1)
 is an approximation of electromagnetic field theory that can be obtained from
Maxwell equations
 active circuit elements: current and voltage sources
 passive circuit elements: resistance, inductance and capacitance
 in direct current, time-harmonic and transient electrical circuit analysis, voltage
is unique and equal to difference of nodal voltages (nodal potentials)
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Summary
 only in the static fields, voltage is identical to the potential difference
(due to conservative nature of static fields, voltage does not depend on the
integration path between any two points)
 in the time-varying fields → voltage and potential difference are not identical;
potential difference between two points is unique;
voltage and induced electromotive force depend on the integration path
 in the transmission line model → the time-varying voltage between two points
depends on the path of integration → voltage is ambiguous
transversal voltage is a special case of voltage equal to the potential difference
 in electrical circuit analysis → voltage is unique and equal to difference of nodal
voltages (nodal potentials)
INDS′11 & ISTET′11 ─ S. Vujević, T. Modrić, D. Lovrić
Thank you!